On the Behavior of Indole-Containing Species Sequestered Within Reverse Micelles at Sub-zero Temperatures

Authors: Munson, Chase A.¹; Kelepouris, Lee²; Baker, Gary A.; Baker, Sheila N.³; Blanchard, Gary J.⁴; Bright, Frank V.⁵

Source: Applied Spectroscopy, Volume 61, Issue 5, Pages 94A-102A and 459-569 (May 2007) , pp. 537-547(11)

Publisher: Society for Applied Spectroscopy

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Abstract:

We report on the effects of temperature (+30 to −100 °C) on the fluorescence from N-acetyl tryptophanamide (NATA) and human serum albumin (HSA) sequestered within Aerosol-OT (AOT) reversed micelles. NATA reports simultaneously from the polar and non-polar side of the reverse micelle interface. As the sample temperature decreases, the relative fraction of NATA molecules associated with the polar side increases. This redistribution process is characterized by ΔH = −14.8 ± 0.6 kJ/mol and ΔS = −54 ± 2 J/(K mol). The activation energy for thermal quenching (E_a,TQ) associated with the polar side NATA molecules is 6.7 kJ/mol before the micelles have shed water and 1.0 kJ/mol after water shedding (below approximately −20 °C). The time-resolved fluorescence intensity decay for tryptophan-214 in HSA is triple exponential. We suggest that these lifetimes arise from three indole residue conformations in equilibrium. Cooling the sample causes a freezing-in of the least quenched conformer; the other conformers are frozen out. The E_a,TQ value for the shortest lifetime component is 6 kJ/mol. The E_a,TQ for the long and intermediate lifetime components are equivalent (∼1.5 kJ/mol).

Keywords: FLUORESCENCE; HUMAN SERUM ALBUMIN; HSA; N-ACETYL TRYPTOPHANAMIDE; NATA; REVERSE MICELLES; AEROSOL OT; AOT; TEMPERATURE

Document Type: Research article

DOI: 10.1366/000370207780807795

Affiliations: 1: Department of Chemistry, Natural Sciences Complex, University at Buffalo, The State University of New York, Buffalo, New York 14260-3000; US Army Research Laboratory, AMSRD-ARL-WM-BD, Aberdeen Proving Ground, Maryland 21005-5069 2: Department of Chemistry, Michigan State University, East Lansing, Michigan 48824-1322; MPI Research, 54943 Main St., Mattawan, Michigan 49071; Chemical Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-61 3: Ionnovations, 704 Gracewood Way, Knoxville, Tennessee 37934 4: Department of Chemistry, Michigan State University, East Lansing, Michigan 48824-1322 5: Department of Chemistry, Natural Sciences Complex, University at Buffalo, The State University of New York, Buffalo, New York 14260-3000

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